1. Field of the Invention
This invention relates to novel devices and methods for making these devices using high T.sub.c superconductors, the superconductors exhibiting superconductivity at temperatures greater than 30.degree. K., and more particularly to substantially planar high T.sub.c SQUID devices and methods for making these devices.
2. Description of the Related Art
Superconductivity is usually defined as the complete loss of electrical resistance of a material at a well defined temperature. It is known to occur in many materials, including about a quarter of the elements of the periodic table and over 1000 alloys and other multi-component systems. Generally, superconductivity is considered to be a property of the metallic state of the material since all known superconductors are metallic under the conditions that cause them to be superconducting. A few normally non-metallic materials, for example, become superconducting under very high pressure, the pressure converting them to metals before they exhibit superconducting behavior.
Superconductors are known to be very attractive for many applications, and in particular high speed switching devices, such as Josephson type switches, high density packaging and circuit layouts. Superconductors are also used in different types of electronic instrumentation, and provide very accurate and sensitive magnetic susceptometers and magnetometers.
While the advantages of superconductors are quite obvious to scientists and engineers, the common disadvantage of superconductive materials is their very low transition temperature. This temperature is often called the critical temperature T.sub.c, and is the temperature above which superconductivity will not exist. Usually, T.sub.c is on the order of a few degrees Kelvin. For many years, the composition having the highest known T.sub.c was Nb.sub.3 Ge which exhibits a T.sub.c of about 23.degree. K. A review of these materials is contained in M. R. Beasley et al, Phys. Today, 37 (10),60 (1984).
In 1986, a significant technical breakthrough was reported by J. G. Bednorz and K. A. Mueller in Z. Phys. B-Condensed Matter, 64 pp. 189-193 (1986). This was the first major improvement in the superconducting transition temperature in the last decade. The materials described by Bendorz and Mueller were transition metal oxides which could include rare earth or near rare earth elements as well as alkaline earth element substitutions. They are layer-like crystalIine structures often characterized by oxygen deficiencies. It is believed that the transition metal must be multi-valent while many choices can be made for the rare earth, near rare earth and alkaline earth elements. Examples of such materials include oxides in the La-Sr-Cu-O and Y-Ba-Cu-O systems. Another publication further describing these materials is J. G. Bednorz et al., Europhysics Letters, 3 (3), pp. 379-385 (1987). The class of materials first described by Bednorz and Mueller will be hereinafter referred to as high T.sub.c superconductors. This is the term generally used by those working in this field to describe these materials, the materials being characterized as transition metal oxides having superconducting transition temperatures greater than about 30.degree. K.
Since the pioneering work of Mueller and Bednorz, there has been considerable technical activity to further develop these superconductors and to provide compositions having even higher critical transition temperatures. Reference is made to co-pending application Ser. No. 24,653 filed Mar. 11, 1987 and assigned to the present assignee, describing a single phase Y-Ba-Cu-O system exhibiting superconductivity at a temperature well above 77.degree. K., and a method for making this composition. A representative composition described in this co-pending application has the formula A.sub.1 M.sub.2 Cu.sub.3 O.sub.y, where A is Y, or a combination of Y, La, Lu, Sc or Yb; M is Ba, or a combination of Ba, Sr or Ca; and y is sufficient to satisfy the valence demands of the composition. Further references describing these high T.sub.c superconductors, and particularly the La-Sr-Cu-O and Y-Ba-Cu-O systems are the following:
Cava et al, Phys. Rev. Letters, 58, 408 (1987); PA0 Chu et al, Phys. Rev. Letters, 58, 405 (1987). PA0 M. Ito et al, Japanese Journal of Applied Physics, 21 No. 6, pp L375-L376, June 1982 PA0 M. Ito et al, Appl. Phys. Lett. 43 (3), p 314, Aug. 1, 1983 PA0 T. Inamura et al, Japanese Journal of Applied Physic, 21, Supplement 21-1, pp. 313-318, 1982.
Another significant advance in the field of high T.sub.c superconductors was the first report of the successful fabrication of films of high T.sub.c compositions and specifically films belonging to the La-Sr-Cu-O and Y-Ba-Cu-O systems. These films were described Mar. 18, 1987 at the meeting of the American Physical society in New York City, and will be further detailed in a paper by R. B. Laibowitz et al submitted for publication.
The general teaching of Laibowitz et al is a vapor deposition technique in which multiple metal sources are used to provide vapor transport of metal atoms to a substrate which is in an oxygen ambient. For example, electron beam heated sources are filled with the desired metals, e.g., La, Sr, and Cu, or Y, Ba, Cu. The rates of evaporation are adjusted to give the nominal desired composition at the substrate. Subsequent annealing in an oxygen atmosphere at about 900.degree. C. is used to provide the desired stoichiometry.
These films and a more detailed description of the fabrication process are described in a co-pending application Ser. No. 27,584 filed Mar. 18, 1987, and assigned to the present assignee. The teaching of the copending application is herein incorporated by reference.
Although many scientific studies have been made concerning these new high T.sub.c superconductors in order to understand the physics and chemistry of these materials, no one heretofore has reported on devices and techniques for making devices using these high T.sub.c superconductors. In particular, there has been no report of the successful operation of devices comprised of these high T.sub.c materials, nor of techniques to make such devices having a substantially planar structure. Generally, superconducting devices utilize multi-layers of different materials and have a non-planar geometry. However, some materials, because of their polycrystalline structure, can include grain boundaries that provide potential barriers for the flow of electrons thereacross and can in this way be used as tunneling devices. Such devices are often called boundary layer Josephson junctions, and are described in the following references:
The devices described in these references occur because of the grain boundaries that result in the deposited films during their preparation. These references do not teach a way to process a deposited film in a manner to controllably produce superconducting and nonsuperconducting regions, and also do not show how to make devices such as SQUIDs.
Accordingly, it is a primary object of the present invention to provide substantially planar devices and methods for making these devices using high T.sub.c superconductor materials.
It is another object of this invention to provide a device that is operable at temperatures in excess of 30.degree. K. and which is fabricated entirely of high T.sub.c superconducting materials.
It is another object of this invention to provide a technique for controllably creating superconducting, normal and insulating portions in a single layer or a bulk sample of high T.sub.c superconductor.
It is another object of this invention to provide 3D device structures in a single layer of high T.sub.c superconducting material, and methods for making these 3D devices.
It is another object of this invention to provide a SQUID device comprised of at least one weak superconducting link formed in high T.sub.c superconducting material.
It is another object of the present invention to provide a DC SQUID device that successfully operates at a temperature in excess of 60.degree. K., and methods for making the same.
It is another object of this invention to provide a SQUID device comprised of high T.sub.c superconducting material, the SQUID device having a substantially planar configuration and operating at temperatures in excess of 60.degree. K.
It is a further object of this invention to provide improved techniques for controllably producing localized regions of a high T.sub.c superconductor which are metallic and normal, (i.e., nonsuperconducting), or insulating).
It is a further object of the present invention to provide a processing technique for locally changing the properties of a high T.sub.c superconductor in such a way as to be able to create devices in the superconductor without the requirement for using different materials and/or multiple layers etc. of the same or different material.